Electronics > Beginners
Why are V(rms) measurements frequency dependant?
Alex Nikitin:
Hmm, unless you really want to use a lot of time and money for purely educational purposes, I see no chance that you can reach your goals the way you've described. I would advise you to buy a good old meter (say, HP3456A has a pretty decent accuracy on AC and you can get it for under $200).
Cheers
Alex
Cerebus:
--- Quote from: Hero999 on August 06, 2018, 02:51:55 pm ---
--- Quote from: sourcecharge on August 06, 2018, 11:34:41 am ---So, just checked the pricing at digikey and the only resistors with a greater tolerance of 0.1 were through hole types, and they were pricey.
Total cost of just the resistor network before tax and shipping, 196 bucks.
:--
--- End quote ---
That's because you've used weird values. 9×10x is not a common resistor value, so it will be expensive, especially in 0.1% tolerance or better.
You would have more luck if you used standard E24 of E96 values. If you divide all of the precision resistor values in that circuit by 5, then it would give you much more widely available resistor values.
--- End quote ---
Actually exactly that series of values is available as a purpose designed divider network from Vishay, the CNS 471 series. Available with ratio tolerances of either 0.03%, 0.05% or 0.1% and tempco tracking of <2.5ppm/C. Datasheet attached. Not cheap, typical one off price (from memory) in the £30-40 GBP region, but that does get you the whole divider network.
David Hess:
--- Quote from: sourcecharge on August 08, 2018, 03:08:45 pm ---That makes sence about the scope dividers, and old scopes used variable trimmer resistors and caps.
--- End quote ---
I am not sure how newer oscilloscopes avoid this and some do not. But newer oscilloscopes tend to only have one switchable high impedance attenuator simplifying things. They pay for this by having to support a much higher input dynamic range.
--- Quote ---I don't think the noise is going to be a problem anymore after seeing the results of today's tests, the input of the LTC1968 would be either between 50mVrms and 500mVrms, or 100mVrms and 200mVrms.
--- End quote ---
Probably not since the RMS converter has a relatively limited dynamic range.
--- Quote ---I really don't like bootstrapping, it limits the frequency range doesn't it?
--- End quote ---
Yes, but not within the frequency range you are considering. Bootstrapping is certainly feasible to 1 MHz and above but it involves a lot of extra complexity so should be avoided unless it is required to remove the need for problematical high impedance attenuators.
--- Quote ---The errors in phase vs frequency of the inverting OP, would have significance, but after measuring the resistors today, it would require balancing capacitors.
--- End quote ---
The high impedance attenuators will always require balancing capacitors. But even your low impedance attenuators may require balancing capacitors in a high precision design. Oscilloscope vertical amplifiers include trims for AC performance in the low impedance stages and you may need to do the same thing.
It may not matter here but usually inverting amplifiers are used to avoid common mode input range and common mode rejection ratio limitations. I do not think that will be an issue for you. Note that the feedback network of the operational amplifiers has all of the same issues as a low impedance divider.
--- Quote ---Anyways, because the capacitors are required, the first idea with just the 2 or 3 Gain OPs and the resistor/capacitor divider network is probably the cheapest, and it could use some variable resistors and caps to offset the tolerances. Maybe also pot the fixed resistors and caps in corona dope.
--- End quote ---
Just be careful that any coating or potting does not create hook errors as described in the Tektronix article I linked.
--- Quote ---That's not really a best solution because those things never stay put. Basically, as a rule of thumb for the "add-on" the divider network would have to be calibrated every time you use it just to be sure the resistors and caps didn't change.
--- End quote ---
0.1% precision over time and temperature is feasible but maybe not for a discrete divider. Hybrid construction can use laser trimmed resistors and capacitors which track each other. A discrete design would require some experimentation to find capacitors and trimmer capacitors which track.
--- Quote ---I'm not sure I'm using an open loop gain OP.
I thought the gain was caused by positive feedback with the non inverting OP, and negative feedback with the inverting OP for unity gain, and attenuation.
Would you provide a rough schematic of what you are talking about?
--- End quote ---
You already drew it; I considered the design you showed with 3 cascaded x10 non-inverting amplifiers. Achieving 0.1% flatness to 100kHz with a x10 gain stage is not trivial and where the open loop gain with frequency becomes a serious problem. There is an example in the Linear Technology application note that I linked of a similar cascaded amplifier.
Kleinstein:
The scope inputs are quite a bit more critical when it comes to frequency response. For the RMS measurement a pure phase shift does not matter - for a scope phase shifts are usually the bigger problem, as they often start to come up well below the transition frequency. One could still a similar construction.
I am not sure how they can get away with less adjustable caps for the dividers. One part can be using ready made programmable divider chips and also having a well reproducible layout, to that fixed caps (measured, tested with prototypes). It also helps that SMT parts usually have lower parasitic capacitance. AFAIK modern scopes often only have the coarse divider before the amplifier and the fine steps behind the amplifier at lower impedance.
Noise is not a big problem at the input of the LTC1968 with some 50 mV-500 mV amplitude. However it still limits the usefulness of lower ranges with more amplification. Unless the input is very low noise (or with an extra bandwidth limit), there is little use for a gain higher than 100.
Kalvin:
The application note from Analog / LInear by Jim Williams "Instrumentation Circuitry Using RMS-to-DC Converters" covers LTC1967/LTC1968/LTC1969 RMS-to_DC converters and gives very useful insight how to apply these devices for high precision measurements from 10 uV upwards. The application note provides also good overview what to expect from these devices, and the application note covers also the 1000X preamplifier design.
http://www.analog.com/media/en/technical-documentation/application-notes/an106f.pdf
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version